Manganese-stabilized austenitic stainless steels for fusion applications

ABSTRACT

An austenitic stainless steel that is comprised of Fe, Cr, Mn, C but no Ni or Nb and minimum N. To enhance strength and fabricability minor alloying additions of Ti, W, V, B and P are made. The resulting alloy is one that can be used in fusion reactor environments because the half-lives of the elements are sufficiently short to allow for handling and disposal.

This invention was developed pursuant to a contract with the U.S.Department of Energy.

This invention relates to austenitic stainless steels that arestabilized with manganese having the base compositionFe-10/18Cr-20/25Mn-0.1/0.3C and minor alloying additions of W, Ti, V, Pand B added for strength. Unlike conventional stainless steels, theycontain no Ni and minimum N, elements that when irradiated in a fusionreactor produce long-lived radioactive isotopes that are difficult todispose of and dangerous to handle.

BACKGROUND

Austenitic Fe-Cr-Ni stainless steels are attractive candidates forfirst-wall and structural materials for magnetic fusion reactors. Steelslike type 316 have good fabricability, strength, ductility and arecommercially available; however, such steel compositions contain certainelements, such as nickel, molybdenum, copper, niobium and nitrogen, thatwhen exposed to radiation form radioactive isotopes that have longhalf-lives. The result of operating a fusion reactor made of this typeof steel would be the conversion of the steel into radioactive materialthat could not be serviced directly by humans nor be easily disposed of.To also serve as a good structural material despite their reducedreactivity, the materials must possess good unirradiated properties,particularly strength, as well as good resistance to adverse propertychanges during irradiation. Therefore, there is a need to developstructural materials for magnetic fusion reactors that either do notconvert or minimally convert to radioactive isotopes of long half-lifeupon exposure to radiation.

SUMMARY OF THE INVENTION

In view of the above need, it is an object of this invention to providestructural materials for fusion reactor applications that do not convertto radioactive isotopes of long half-life upon exposure to radiation.

Another object of this invention is to provide an austenitic alloy thathas no Ni or Nb and as little N as possible.

It is another object of this invention to provide steels that do notcontain nitrogen, molybdenum, copper, niobium or nickel but thatotherwise exhibit an austenite, face-centered-cubic crystal structure.

It is a further object of this invention to provide structural materialfor fusion reactors that are strong, ductile, inexpensive and easy tofabricate. Additional objects, advantages and novel features of theinvention will be set forth in part in the description which follows,and in part will become apparent to those skilled in the art uponexamination of the following or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, as embodied and broadly describedherein, the composition of matter of this invention may comprise astainless steel of iron and chromium and a sufficient amount ofmanganese and carbon to form a face centered cubic austeniticcrystalline structure. For strength and ductility the base alloy shouldinclude sufficient titanium in the presence of tungsten or vanadium orcombinations thereof, to form small, uniform precipitates. A betteralloy further includes minor additions of boron and phosphorous tosupport the strengthening effect of the precipitates. If the compositionof matter is not to be used in a fusion reactor, the invention may alsocomprise small amounts of nickel and nitrogen. One advantage fornon-fusion applications relative to commercial Fe-Cr-Ni structuralsteels is that less nickel, which is expensive and in short supply, isneeded. Another is that nitrogen, which is difficult to exclude, ispermitted. The alloys of this invention have the distinct advantage forfusion applications of being inexpensive, easy to maintain and safer todispose of after exposure to a radiation environment than currentlyavailable related steels or alloys.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a modified Schaeffler diagram used to determine thecompositions of Fe-Cr-Mn-C alloys having the austenitic phase structure.

FIG. 2 is a diagram showing the yield stress of a simplemanganese-stabilized steel in the solution annealed condition and 20%cold-worked condition as compared with type 316 stainless steel in thesame conditions.

FIG. 3 is a diagram showing the ultimate tensile strength for a simplemanganese-stabilized steel and type 316 stainless steel in both thesolution-annealed and 20% cold-worked condition.

FIG. 4 is a diagram showing the ductility, as measured by totalelongation, of simple manganese-stabilized steel and type 316 stainlesssteel in solution annealed and 20% cold-worked conditions.

FIG. 5 is a diagram that shows yield stress of five minor solutemodified manganese-stabilized steels and the type 316 stainless steel inthe solution-annealed condition.

FIG. 6 is a diagram that shows ultimate tensile strength of five minorsolute modified manganese-stabilized steels and type 316 stainless steelin the solution-annealed condition.

FIG. 7 is a diagram that shows the ductility, in terms of totalelongation, of five minor solute modified manganese-stabilized steelsand type 316 stainless steel in the solution-annealed condition.

FIG. 8 is a diagram that shows yield stress of five minor solutemodified manganese-stabilized steels and type 316 stainless steel in the20% cold-worked condition.

FIG. 9 is a diagram that shows ultimate tensile strength of five minorsolute modified manganese-stabilized steels and type 316 stainless steelin the 20% cold-worked condition.

FIG. 10 is a diagram that shows the ductility, in terms of totalelongation, of five minor solute modified manganese-stabilized steelsand type 316 stainless steel in the 20% cold-worked condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Radioactive decay values have been calculated for the various elementsthat generally comprise stainless steels. The radioactive decay behaviorfor steels can be classified into categories of low-activation materialsand fast induced-radioactivity decay materials. The term"low-activation" ideally describes materials that would allow hands-onmaintenance immediately after shutdown, and only materials like pure Vor SiC can be classified as such. The term "fast induced-radioactivitydecay" (FIRD) best describes engineering materials that would not allowhands-on maintenance, but could be disposed of by shallow land burialafter reactor decommissioning. One approach to alloy design for fusionreactors is the substitution of standard steel alloying elements thatproduce long-lived radioactive isotopes with FIRD elements; however,development of an such a steel with an austenite structure, that iseasily fabricable and has low- and high-temperature strength has beendifficult.

The strategy for the development of FIRD alloys has been the replacementof elements like Mo, Nb and Ni in Fe-Cr-Ni-Mo steels such as type 316with elements like Mn, W, Ti, V, Ta Si and C. The development of thealloys of this invention began with a base alloy having iron, chromium,manganese and carbon. The Mn and C, used in the place of Ni, give thealloy its face-centered cubic austenite structure. Studies done on aseries of alloys set forth in Table 1 provided phase information thatled to the development of a modified Schaeffler diagram, FIG. 1. Theoriginal Schaeffler diagram was developed for Fe-Cr-Ni alloys, andobservations on the alloys of Table 1 indicated that this diagram didnot predict the constituent phases of Fe-Cr-Mn-C alloys after annealing.As a result of the work on the alloys of Table 1, the new diagram(FIG. 1) was developed which does predict the appropriate ranges ofelements for a stable austenitic alloy.

                  TABLE 1                                                         ______________________________________                                        Composition, wt %                                                             Alloy Mn     Cr     C      Ti  W   V   P      B   N                           ______________________________________                                        0     13.4   15.0   0.069                     0.001                           1     14.2   14.8   0.014                     0.001                           2     17.1   15.2   0.056                     0.001                           3     13.4   10.0   0.089                     0.002                           4     18.9   9.9    0.093                     0.002                           5     12.4   15.3   0.180                     0.002                           6     14.3   16.0   0.180                     0.003                            7*   18.8   14.8   0.380                     0.005                           8     17.7   20.1   0.130                     0.003                           9     17.6   20.2   0.260                     0.006                           10    19.9   10.0   0.081                     0.005                           11    20.0   11.9   0.084                     0.009                           12*   20.1   12.0   0.180                     0.008                           13    19.1   14.0   0.088          0.003      0.013                           14    19.8   15.9   0.170          0.003      0.001                           ______________________________________                                         *entirely austenitic                                                     

One selected base alloy of Fe-20Mn-12Cr-0.25C had the desired structurewith strength comparable to type 316 stainless steel as shown in FIGS. 3and 4. The next step in development of the invention was an improvementin both strength and irradiation resistance. This required fine tuningthe alloy composition with minor element additions and combinations toproduce fine, stable MC precipitation without upsetting the austenitestability of the base alloy or degrading its properties. Alloys preparedin this study are set forth in Table 2. Titanium is one element thatmust be present for precipitate formation along with either tungsten orvanadium or a combination of the two. Although precipitate formationoccurs when Ti and V additions are made, the necessary interactionbetween dislocations and the fine precipitate particles, which is thebasis of high temperature strength, is not optimum. Alloying with boronand phosphorous resulted in their interaction with Ti and V or Ti, V,and W to cause the precipitates of TiC, WC and VC to be small anduniform and to interact with dislocations and grain boundaries so thatprecipitates could pin them, thus producing a metal that is strong athigh temperatures.

                                      TABLE 2                                     __________________________________________________________________________             Composition, wt %                                                    Alloy    Mn Cr C  Ti W  V  P  B   N                                           __________________________________________________________________________    15       20.5                                                                             11.8                                                                             0.240    0.01                                                                             0.004  0.002                                       MnCrC                                                                         16       20.5                                                                             11.7                                                                             0.250                                                                            0.11                                                                             0.09                                                                             0.01                                                                             0.003  0.003                                       MnCrCTi                                                                       17       20.5                                                                             11.8                                                                             0.230 0.83                                                                             0.01                                                                             0.004  0.003                                       MnCrCW                                                                        18       21.1                                                                             11.7                                                                             0.250                                                                            0.12                                                                             0.77                                                                             0.01                                                                             0.003  0.003                                       MnCrCTiW                                                                      19       20.5                                                                             11.8                                                                             0.240                                                                            0.10  0.01                                                                             0.034                                                                            0.005                                                                             0.034                                       MnCrCTiPB                                                                     20       20.8                                                                             11.8                                                                             0.220                                                                            0.10  0.10                                                                             0.033                                                                            0.005                                                                             0.004                                       MnCrCTiVPB                                                                    21       20.4                                                                             11.7                                                                             0.250                                                                            0.10                                                                             1.10                                                                             0.10                                                                             0.027                                                                            0.500                                                                             0.004                                       MnCrCTiWVPB                                                                   22       21.0                                                                             13.8                                                                             0.140    0.01                                                                             0.004  0.002                                       MnCrCTiP                                                                      23       20.9                                                                             13.6                                                                             0.110                                                                            0.09                                                                             1.28                                                                             0.02                                                                             0.004                                                                            0.001                                                                             0.003                                       MnCrCTiPB                                                                     24       21.0                                                                             13.6                                                                             0.190                                                                            0.11                                                                             1.27                                                                             0.10                                                                             0.028                                                                            0.006                                                                             0.003                                       25       20.9                                                                             11.9                                                                             0.075    0.01                                                                             0.004  0.002                                       26       20.8                                                                             11.7                                                                             0.096                                                                            0.11                                                                             1.25                                                                             0.02                                                                             0.004                                                                            0.001                                                                             0.002                                       27       20.9                                                                             11.6                                                                             0.078                                                                            0.11                                                                             1.26                                                                             0.10                                                                             0.037                                                                            0.006                                                                             0.003                                       28       18.8                                                                             11.7                                                                             0.240                                                                            0.33                                                                             1.98                                                                             0.01                                                                             0.003                                                                            0.001                                                                             0.008                                       29       19.2                                                                             11.7                                                                             0.240                                                                            0.34                                                                             1.94                                                                             0.01                                                                             0.044                                                                            0.008                                                                             0.006                                       30       19.6                                                                             11.8                                                                             0.250                                                                            0.09                                                                             1.96                                                                             0.01                                                                             0.043                                                                            0.008                                                                             0.014                                       31       19.0                                                                             11.8                                                                             0.250                                                                            0.09                                                                             3.15                                                                             0.01                                                                             0.041                                                                            0.008                                                                             0.008                                       __________________________________________________________________________

EXAMPLE 1

A nominally Fe-12Cr-20Mn-0.25C steel was melted, fabricated and tensiletested. FIGS. 2 through 4 compare the tensile properties of this simplesteel, labeled MnCrC, in the solution-annealed and 20% cold-workedconditions (common conditions for using such a steel) with type 316stainless steel in the same conditions. The yield stress of themanganese-stabilized steel in both conditions is equivalent to that of316 stainless steel, as shown in FIG. 2. Because of higherwork-hardening characteristics imparted by manganese, the high manganesesteel achieves a higher ultimate tensile strength for both conditions,as shown in FIG. 3. Despite this higher work hardening capability, thehigh manganese steel still has equivalent or better ductility than type316, as measured by total elongation both in solution-annealed and inthe cold-worked condition, as shown in FIG. 4. The results indicate thatan adequate austenitic base Fe-Cr-Mn-C alloy can be obtained using theinformation developed in the modified Schaeffler diagram.

EXAMPLE 2

The next objective was to improve the strength of the new alloys bymaking further minor element additions and combinations to the basecomposition. This was accomplished by adding Ti, W, V, P and B to thenominally Fe-12Cr-20Mn-0.25C base composition. The alloy combinationsthat were melted, fabricated and tensile tested are shown as alloysnumbered 15 through 21 in Table 2. The lettered alloy designationsindicate the alloying elements added to the iron. For example, MnCrCTiWindicates that Mn, Cr, C, Ti and W were added to the iron. In preparingthe alloys, the targeted amounts in wt % of the various elements were 12for Cr, 20 for Mn, 0.25 for C, 0.10 for Ti, 1.0 for W, 0.035 for P and0.005 for B, although the actual amounts varied slightly in the finalcompositions, as shown in Table 2. In Table 3, the room temperaturetensile properties for seven alloys, including the MnCrC steel, aregiven along with similar results for a heat of type 316 steel. Thesteels were tested in two solution-annealed conditions and in the 20%cold-worked condition. The type 316 steel was tested in one of thesolution-annealed conditions and in the 20% cold-worked condition.

                  TABLE 3                                                         ______________________________________                                                   Strength, MPa                                                                             Elongation, %                                          Alloy       YS        UTS     Uniform  Total                                  ______________________________________                                                   Solution Annealed 1 h 1050° C.                              MnCrC       220       798     55.4     56.6                                   MnCrCTi     279       927     49.7     53.0                                   MnCrCW      267       803     57.1     59.9                                   MnCrCTiW    302       918     53.8     56.9                                   MnCrCTiPB   288       935     52.2     55.6                                   MnCrCTiVPB  275       935     51.0     53.9                                   MnCrCTiWVPB 304       915     54.9     57.5                                   316 SS      236       586     54.3     58.2                                              Solution Annealed 2 h 1150° C.                              MnCrC       233       766     53.4     55.1                                   MnCrCTi     258       891     53.5     56.4                                   MnCrCW      247       761     55.4     57.0                                   MnCrCTiW    258       882     54.5     57.2                                   MnCrCTiPB   271       891     52.8     54.2                                   MnCrCTiVPB  221       859     49.6     50.4                                   MnCrCTiWVPB 264       869     59.9     61.7                                              20% Cold Worked                                                    MnCrC       815       1086    14.1     16.0                                   MnCrCTi     954       1160    10.7     13.0                                   MnCrCW      784       1057    17.6     20.0                                   MnCrCTiW    980       1168     6.6      9.5                                   MnCrCTiPB   946       1158    10.4     12.1                                   MnCrCTiVPB  862       1126    11.4     13.1                                   MnCrCTiWVPB 915       1114    11.3     13.6                                   316 SS      739       807     11.5     17.4                                   ______________________________________                                    

The tensile results given in Table 3 show that for the high manganesesteels the strength of the steels solution annealed one hour at 1050° C.generally exceeded those of the same steels annealed two hours at 1150°C. With one exception, after the one hour anneal at 1050° C., the yieldstress and especially the ultimate tensile strength of themanganese-stabilized stainless steels exceeded those of thenickel-stabilized type 316 stainless steel. The exception was the yieldstress for the base MnCrC steel, which was slightly lower than the yieldstress for the type 316 stainless steel. Similarly, the strengthmeasurements of manganese-stabilized steels in the 20% cold-workedconditions exceeded those for the type 316 stainless steel.

In the solution-annealed condition, the ductility as measured by theuniform and total elongations of the manganese-stabilized steels wereequivalent to those for 316 stainless steel. Equivalent ductility wasalso observed for most of the alloys in the cold-worked condition. Theonly exception was the MnCrCTiW alloy, which had the lowest uniform andtotal elongations, although these values would still indicate adequateductility.

A comparison of the room temperature tensile data in Table 3 formanganese-stabilized steels shows the effectiveness of the combinationof Ti, W, V, B and P on the strength and ductility of the Fe-Cr-Mn-Cbase composition. The steels were further tested over the temperaturerange of room temperature to 600° C. In FIGS. 5-10, the tensileproperties for the five strongest steels are compared with those fortype 316 stainless steel. The strength results clearly show thesuperiority of the manganese-stabilized steels in both thesolution-annealed conditions, as shown in FIGS. 5 and 6, and thecold-worked condition, as shown in FIGS. 8 and 9. Despite this strengthsuperiority, the ductility is equivalent or better than that for 316stainless steel in the solution-annealed condition, as shown in FIG. 7.In the cold-worked condition, the 316 stainless steel has a higher totalelongation below 200° C., but at higher temperatures, themanganese-stabilized steels have equivalent or better ductility as shownin FIG. 10. These observations on ductility are important becausenormally strength and ductility are trade-offs. The new steels,therefore, represent a significant gain in strength that does not comeat the expense of ductility. These properties give these new steels thepotential for non-fusion applications from 20°-600° C. in addition totheir application as FIRD steels for fusion.

The results of this work indicate that favorable manganese stabilizedstainless steel can be achieved with composition in wt % of 10-18Cr,20-25Mn, 0.1-0.3C, W, Ti, V, B and P. Also permitted in small amounts isnickel, as well as nitrogen that is unavoidable due to presence in theatmosphere during most commercial processing, without significant harmto strength and ductility. The latter alloys with less stringentcompositional limitations would not be used in fusion reactorenvironments due to the long half-lives of radioactive nitrogen andnickel but would be suitable for other non-fusion uses as a cheaper,stronger substitute for type 316 stainless steels from room temperatureto about 600° C.

We claim:
 1. A composition of matter of austenitic stainless steelconsisting essentially of iron, chromium and a sufficient amount ofmanganese and carbon to form a face centered cubic austeniticcrystalline structure.
 2. The composition of claim 1 wherein saidcomposition consists essentially of in wt % 10-18Cr, 20-25Mn, 0.05-0.3Cand the balance Fe.
 3. The composition of claim 1 further consistingessentially of sufficient titanium in the presence of tungsten orvanadium or combinations thereof, to form small, uniform precipitatesfor strength and ductility.
 4. The composition of claim 3 wherein saidcomposition further consists essentially of in weight % 0.1-0.3 Ti, 1-3W, 0.1-0.3 V.
 5. The composition of claim 3 further consistingessentially of sufficient amounts of boron and phosphorous to supportthe strengthening effects of said precipitates.
 6. The composition ofclaim 5 wherein said composition further consists essentially of inweight % 0.005-0.01 B and 0.03-0.08 P.